queryTagHistory and DST

Ignition
1

How does Ignition handle requesting data via queryTagHistory that spans a transition into or out of daylight savings time? I'm not seeing one hour less into DST and an extra hour out of DST, as I would expect.

2

Ignition datetime values are really instances of java.util.Date, which are UTC under the hood. Always. They display in a local time zone when printed (stringified), and are constructed in local time zones when parsed. At all other times, they are zone-less. You should recheck how you are generating your start/end boundaries.

3

I'm specifying startDate and endDate with Ignition DateTime objects.

4

Of course you are, because that's what the function requires. How are you getting those Datetime objects? Show your code.

5 
# ./Controls/Start & ./Controls/End = Perspective DateTime Inputs

# props.custom.start (expr) = 
# if({this.custom.historical}, {./Controls/Start.props.value}, addMinutes({this.custom.end}, {this.custom.minutes}*-1))


props = self.custom
props.wait = True
query_tags = []

for tag in props.tags:
    query_tags.append(str(tag.path))

data = system.tag.queryTagHistory(
    paths = query_tags,
    startDate = props.start,
    endDate = props.end,
    columnNames = query_tags,
    returnSize = props.returnSize,
    intervalSeconds = props.aggregation,
    aggregationMode = props.aggregationMode,
    noInterpolation = False,
    ignoreBadQuality = True,
    validateSCExec = False)

self.refreshLayout()

props.data = data
props.wait = False
6

Ok, you have custom datetime properties. How are you writing to those before the history query runs?

Some other script? Datetime input components?

7

Yes. See the top comment in the above code for reference. I'm using DateTime Input components. The value is being read from a custom property via expression. Depending on the mode, I'm either taking the value directly or using the end time and deducting a specified number of minutes using the addMinutes expression function.

8

What does your session timezone property show? (That is what your datetime inputs will use.) The date arithmetic for non-historical will act on the UTC datetime objects, so I hope you don't expect that to respect your time zones.

9

That's It. It does seem the perspective project was changed to UTC at some point. I understand now, thank you. I should have checked there first. When the zone is set correctly, it does, in fact, return the extra hour when transitioning from PDT to PST.

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